Remove undefined from optional properties when inferring to index signatures (#43086)

* Remove undefined from optional properties when inferring to index signatures

* Add tests

tests/cases/compiler/inferenceOptionalPropertiesToIndexSignatures.ts

@@ -0,0 +1,27 @@
+// @strict: true
+// @target: esnext
+
+declare function foo<T>(obj: { [x: string]: T }): T;
+
+declare const x1: { a: string, b: number };
+declare const x2: { a: string, b: number | undefined };
+declare const x3: { a: string, b?: number };
+declare const x4: { a: string, b?: number | undefined };
+
+let a1 = foo(x1);  // string | number
+let a2 = foo(x2);  // string | number | undefined
+let a3 = foo(x3);  // string | number
+let a4 = foo(x4);  // string | number
+
+// Repro from #43045
+
+const param2 = Math.random() < 0.5 ? 'value2' : null;
+
+const obj = {
+    param1: 'value1',
+    ...(param2 ? {param2} : {})
+};
+
+const query = Object.entries(obj).map(
+    ([k, v]) => `${k}=${encodeURIComponent(v)}`
+).join('&');